Datasheet

10
LTC1485
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The termination resistors are used to generate a DC bias
which forces the receiver output to a known state, in this
case a logic 0. The first method consumes about 208mW
and the second about 8mW. The lowest power solution is
to use an AC termination with a pull-up resistor. Simply
swap the receiver inputs for data protocols ending in
logic 1.
Fault Protection
All of LTC’s RS485 products are protected against ESD
transients up to 2kV using the human body model
(100pF, 1.5kΩ). However, some applications need more
protection. The best protection method is to connect a
bidirectional TransZorb
®
from each line side pin to ground
(Figure 15).
A TransZorb is a silicon transient voltage suppressor that
has exceptional surge handling capabilities: fast response
1485 F15
120Ω
DRIVER
A
B
time and low series resistance. They are available from
General Semiconductor Industries and come in a variety
of breakdown voltages and prices. Be sure to pick a
breakdown voltage higher than the common-mode volt-
age required for your application (typically 12V). Also,
don’t forget to check how much the added parasitic
capacitance will load down the bus.
Figure 15. ESD Protection with TransZorbs
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TransZorb is a registered trademark of General Instruments, GSI
RS232 Receiver
1485 TA02
RECEIVER
RS232
IN
5.6k
RX
RS232 to RS485 Level Translator with Hysteresis
1485 TA03
120Ω
DRIVER
A
B
220k
10k
RS232
IN
HYSTERESIS = 10k •  V
A
– V
B
 /R ≈ 19 (kΩ • VOLT)/R
5.6k